Power conversion apparatus and synchronous rectification controller thereof

ABSTRACT

A power conversion apparatus and a synchronous rectification controller thereof are provided. At least one of a differentiation operation and an integration operation is performed on a drain voltage signal of a synchronous rectification transistor. According to at least one of a differential signal obtained by performing the differential operation and an integral signal obtained by performing the integral operation, it is determined whether to turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to a turn-on threshold voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwanese applicationno. 110130730, filed on Aug. 19, 2021. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND Technical Field

The disclosure relates to a power apparatus. In particular, thedisclosure relates to a power conversion apparatus and a synchronousrectification controller thereof.

Description of Related Art

A power conversion apparatus is an indispensable component in modernelectronic apparatuses. In a power conversion apparatus based on pulsewidth modulation (PWM) control, the power conversion apparatus typicallyhas a rectifier diode at a secondary side. Since power consumption ofthe rectifier diode is relatively large in a turn-on state, asynchronous rectification transistor with a lower on-resistance may beemployed in place of the rectifier diode. Under such an architecture, asynchronous rectification controller is also needed to control thesynchronous rectification transistor at the secondary side to be turnedon or turned off.

When inductor-current is discharged to 0 at the secondary side of thepower conversion apparatus, oscillation may accordingly occurs in adrain voltage of the synchronous rectification transistor. In theconventional art, it may be determined whether to turn on thesynchronous rectification transistor by detecting a grade of a slope ofthe dropping drain voltage, to avoid turning on the synchronousrectification transistor during the inductor-capacitor oscillationperiod, which causes current reflux at an output terminal of the powerconversion apparatus, thus causing damage to the power conversionapparatus and reducing the safety of use of the power conversionapparatus. However, as an operating frequency of current power supplysystems increases, the falling slope of the drain voltage of thesynchronous rectification transistor during the inductor-currentdischarge period becomes closer to the slope during theinductor-capacitor oscillation period gradually. As such, the turn-ontiming of the synchronous rectification transistor may no longer beaccurately determined by the above manner of determination.

SUMMARY

The disclosure provides a power conversion apparatus and a synchronousrectification controller thereof, which effectively increases accuracyin controlling switching of a synchronous rectification transistor to aturn-on state.

The synchronous rectification controller of the disclosure is configuredto control a turn-on state of a synchronous rectification transistor.The synchronous rectification controller includes a turn-on controlcircuit and a turn-off control circuit. The turn-on control circuit iscoupled to a drain terminal of the synchronous rectification transistor,performs at least one of a differentiation operation and an integrationoperation on a drain voltage signal of the synchronous rectificationtransistor, and determines whether to turn on the synchronousrectification transistor at a next time when the drain voltage signal isless than or equal to a turn-on threshold voltage according to at leastone of a differential signal obtained by performing the differentiationoperation and an integral signal obtained by performing the integrationoperation. The turn-off control circuit is coupled to the drain terminalof the synchronous rectification transistor, compares the drain voltagesignal with a turn-off threshold voltage, and turns off the synchronousrectification transistor when the drain voltage signal is greater thanthe turn-off threshold voltage.

In an embodiment of the disclosure, if the drain voltage signal isgreater than or equal to a predetermined voltage, and a signal value ofthe differential signal is equal to 0 for a predetermined time during aperiod in which the drain voltage signal is greater than or equal to thepredetermined voltage, the turn-on control circuit turns on thesynchronous rectification transistor at the next time when the drainvoltage signal is less than or equal to the turn-on threshold voltage;and if the drain voltage signal is not greater than or equal to thepredetermined voltage, or the signal value of the differential signal isnot equal to 0 for the predetermined time during the period in which thedrain voltage signal is greater than or equal to the predeterminedvoltage, the turn-on control circuit does not turn on the synchronousrectification transistor at the next time when the drain voltage signalis less than or equal to the turn-on threshold voltage.

In an embodiment of the disclosure, if a signal value of thedifferential signal is greater than or equal to a predeterminedderivative, and the signal value of the differential signal ismaintained at 0 for a predetermined time after being reduced fromgreater than or equal to the predetermined derivative to 0, the turn-oncontrol circuit turns on the synchronous rectification transistor at thenext time when the drain voltage signal is less than or equal to theturn-on threshold voltage; and if the signal value of the differentialsignal is not greater than or equal to the predetermined derivative, orthe signal value of the differential signal is not maintained at 0 forthe predetermined time after being reduced from greater than or equal tothe predetermined derivative to 0, the turn-on control circuit does notturn on the synchronous rectification transistor at the next time whenthe drain voltage signal is less than or equal to the turn-on thresholdvoltage.

In an embodiment of the disclosure, if the drain voltage signal isgreater than or equal to a predetermined voltage, and a signal value ofthe integral signal is greater than or equal to a predetermined integralduring a period in which the drain voltage signal is greater than orequal to the predetermined voltage, the turn-on control circuit turns onthe synchronous rectification transistor at the next time when the drainvoltage signal is less than or equal to the turn-on threshold voltage;and if the drain voltage signal is not greater than or equal to thepredetermined voltage, or the signal value of the integral signal is notgreater than or equal to the predetermined integral during the period inwhich the drain voltage signal is greater than or equal to thepredetermined voltage, the turn-on control circuit does not turn on thesynchronous rectification transistor at the next time when the drainvoltage signal is less than or equal to the turn-on threshold voltage.

In an embodiment of the disclosure, if a signal value of thedifferential signal is greater than or equal to a predeterminedderivative and a signal value of the integral signal is greater than orequal to a predetermined integral during a period in which the drainvoltage signal is greater than or equal to a predetermined voltage, theturn-on control circuit turns on the synchronous rectificationtransistor at the next time when the drain voltage signal is less thanor equal to the turn-on threshold voltage; and if the signal value ofthe differential signal is not greater than or equal to thepredetermined derivative or the signal value of the integral signal isnot greater than or equal to the predetermined integral during theperiod in which the drain voltage signal is greater than or equal to thepredetermined voltage, the turn-on control circuit does not turn on thesynchronous rectification transistor at the next time when the drainvoltage signal is less than or equal to the turn-on threshold voltage.

In an embodiment of the disclosure, the predetermined integral is equalto an integral value of the drain voltage signal in a predetermined timeduring the period in which the drain voltage signal is greater than orequal to the predetermined voltage.

In an embodiment of the disclosure, the synchronous rectificationcontroller further includes a logic circuit. The logic circuit iscoupled to the turn-on control circuit, the turn-off control circuit,and a gate terminal of the synchronous rectification transistor, and iscontrolled by the turn-on control circuit and the turn-off controlcircuit to generate a synchronous rectification control signal to thegate terminal of the synchronous rectification transistor.

In an embodiment of the disclosure, the logic circuit includes an SRflip-flop. A set terminal and a reset terminal of the SR flip-flop arerespectively coupled to the turn-on control circuit and the turn-offcontrol circuit, and an output terminal of the SR flip-flop is coupledto the gate of the synchronous rectification transistor terminal.

In an embodiment of the disclosure, the turn-off threshold voltage isgreater than the turn-on threshold voltage.

The disclosure also provides a power conversion apparatus, including atransformer, a synchronous rectification transistor, and a synchronousrectification controller. The transformer has a primary side and asecondary side. A first terminal of the primary side is configured toreceive an input voltage, and a first terminal of the secondary side isconfigured to provide an output voltage to a load. A drain terminal ofthe synchronous rectification transistor is coupled to a second terminalof the secondary side, and a source terminal of the synchronousrectification transistor is coupled to a ground terminal. Thesynchronous rectification controller is coupled between the drainterminal and a gate terminal of the synchronous rectificationtransistor, and is configured to control a turn-on state of thesynchronous rectification transistor. The synchronous rectificationcontroller includes a turn-on control circuit and a turn-off controlcircuit. The turn-on control circuit is coupled to the drain terminal ofthe synchronous rectification transistor, performs at least one of adifferentiation operation and an integration operation on a drainvoltage signal of the synchronous rectification transistor, anddetermines whether to turn on the synchronous rectification transistorat a next time when the drain voltage signal is less than or equal to aturn-on threshold voltage according to at least one of a differentialsignal obtained by performing the differentiation operation and anintegral signal obtained by performing the integration operation. Theturn-off control circuit is coupled to the drain terminal of thesynchronous rectification transistor, compares the drain voltage signalwith a turn-off threshold voltage, and turns off the synchronousrectification transistor when the drain voltage signal is greater thanthe turn-off threshold voltage.

Based on the foregoing, the turn-on control circuit in the embodimentsof the disclosure may perform at least one of a differentiationoperation and an integration operation on the drain voltage signal ofthe synchronous rectification transistor, and determine whether to turnon the synchronous rectification transistor at the next time when thedrain voltage signal is less than or equal to the turn-on thresholdvoltage according to at least one of the differential signal obtained byperforming the differentiation operation and the integral signalobtained by performing the integration operation. Accordingly, bydetermining the change of the drain voltage signal of the synchronousrectification transistor with the differential signal and the integralsignal, the inductor-current discharge period and the inductor-capacitoroscillation period can be accurately distinguished, thus effectivelyincreasing the accuracy in controlling the switching of the synchronousrectification transistor to the turn-on state.

To make the aforementioned more comprehensible, several embodimentsaccompanied with drawings are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate exemplaryembodiments of the disclosure and, together with the description, serveto explain the principles of the disclosure.

FIG. 1 is a schematic circuit block diagram illustrating a powerconversion apparatus shown according to an embodiment of the disclosure.

FIG. 2 is a schematic circuit diagram illustrating a synchronousrectification controller according to an embodiment of the disclosure.

FIG. 3 is a schematic circuit diagram illustrating a synchronousrectification controller according to another embodiment of thedisclosure.

FIG. 4 is a schematic signal timing diagram of a synchronousrectification controller according to an embodiment of the disclosure.

FIG. 5 is a schematic signal timing diagram of a synchronousrectification controller according to another embodiment of thedisclosure.

FIG. 6 is a schematic circuit diagram illustrating a synchronousrectification controller according to yet another embodiment of thedisclosure.

FIG. 7 is a schematic signal timing diagram of a synchronousrectification controller according to the yet another embodiment of thedisclosure.

FIG. 8 is a schematic circuit diagram illustrating a synchronousrectification controller according to still another embodiment of thedisclosure.

FIG. 9 is a schematic signal timing diagram of a synchronousrectification controller according to the still another embodiment ofthe disclosure.

DESCRIPTION OF THE EMBODIMENTS

In order to make the disclosure more comprehensible, embodiments areparticularly provided below as examples according to which thedisclosure can be reliably carried out. In addition, wherever possible,elements/members/steps labeled with the same reference numerals in thedrawings and embodiments denote the same or similar parts.

FIG. 1 is a schematic circuit block diagram illustrating a powerconversion apparatus shown according to an embodiment of the disclosure.In this embodiment, a power conversion apparatus 10 has a flybackarchitecture, but is not limited to this. In other embodiments, thepower conversion apparatus may also have, for example, a push-pull,forward, half-bridge, or full-bridge architecture or other types ofarchitecture. Operation of the power conversion apparatus of otherarchitectures may be deduced from the operation of this embodiment.

The power conversion apparatus 10 includes a transformer T, asynchronous rectification transistor MSR, a synchronous rectificationcontroller 102, a power switch Mp, and a pulse width modulation signalgenerator 110, but is not limited thereto. The transformer T includes aprimary side Np and a secondary side Ns. A first terminal (e.g., acommon-polarity terminal, i.e., a dotted terminal) of the primary sideNp is configured to receive an input voltage VIN, and a first terminal(e.g., an opposite-polarity terminal, i.e., non-dotted terminal) of thesecondary side Ns is configured to provide an output voltage VOUT to aload RL (e.g., an electronic device) and charge a capacitor Co.Nonetheless, the disclosure is not limited thereto.

A first terminal of the power switch Mp is coupled to a second terminal(e.g., an opposite-polarity terminal) of the primary side Np, a secondterminal of the power switch Mp is coupled to a second ground terminalGND2, and a control terminal of the power switch Mp is coupled to thepulse width modulation signal generator 110 to receive a pulse widthmodulation signal Spwm. The pulse width modulation signal generator 110may generate and adjust the pulse width modulation signal Spwm accordingto the state of the load RL (or the power supply requirement).

A drain terminal of the synchronous rectification transistor MSR iscoupled to a second terminal (e.g., a common-polarity terminal) of thesecondary side Ns, and a source terminal and a body terminal of thesynchronous rectification transistor MSR are coupled to a first groundterminal GND1. A parasitic diode Dr is present between the drainterminal and the body terminal of the synchronous rectificationtransistor MSR. In an embodiment of the disclosure, the synchronousrectification transistor MSR may be an N-type metal-oxide-semiconductorfield-effect transistor. Nonetheless, the disclosure is not limitedthereto and may be determined depending on actual application or designrequirements. The synchronous rectification controller 102 is coupled tothe drain terminal of the synchronous rectification transistor MSR toreceive a drain voltage signal VD. According to the voltage level of thedrain voltage signal VD, the synchronous rectification controller 102may correspondingly generate a synchronous rectification control signalVG to a gate terminal of the synchronous rectification transistor MSR tocontrol a turn-on state of the synchronous rectification transistor MSR.

The synchronous rectification controller 102 includes a turn-on controlcircuit 104 and a turn-off control circuit 106. The turn-on controlcircuit 104 and the turn-off control circuit 106 are coupled to thedrain terminal of the synchronous rectification transistor MSR.

The turn-on control circuit 104 may perform at least one of adifferentiation operation and an integration operation on the drainvoltage signal VD, and determine whether to turn on the synchronousrectification transistor MSR at the next time when the drain voltagesignal VD is less than or equal to a turn-on threshold voltage Vonthaccording to at least one of a differential signal obtained byperforming the differentiation operation and an integral signal obtainedby performing the integration operation. In addition, the turn-offcontrol circuit 106 may compare the drain voltage signal VD with aturn-off threshold voltage Voffth, and the turn-off control circuit 106may turn off the synchronous rectification transistor MSR when the drainvoltage signal VD is greater than the turn-off threshold voltage Voffth.The turn-off threshold voltage Voffth is greater than the turn-onthreshold voltage Vonth.

Accordingly, by determining the change of the drain voltage signal VD ofthe synchronous rectification transistor MSR with at least one of thedifferential signal and the integral signal of the drain voltage signalVD, it can be accurately distinguished whether the secondary side of thepower conversion apparatus 10 is in an inductor-current discharge periodor an inductor-capacitor oscillation period, thus effectively increasingthe accuracy in controlling the switching of the synchronousrectification transistor to the turn-on state.

In some embodiments, the synchronous rectification controller 102 mayalso include a logic circuit. The logic circuit is coupled to theturn-on control circuit 104, the turn-off control circuit 106, and thegate terminal of the synchronous rectification transistor MSR, and iscontrolled by the turn-on control circuit 104 and the turn-off controlcircuit 106 to generate the synchronous rectification control signal tothe gate terminal of the synchronous rectification transistor. As shownin FIG. 2 , in this embodiment, the logic circuit is implemented as, forexample but not limited to, an SR flip-flop 202. A set terminal S and areset terminal R of the SR flip-flop 202 are respectively coupled to theturn-on control circuit 104 and the turn-off control circuit 106, and anoutput terminal Q of the SR flip-flop 202 is coupled to the gateterminal of the synchronous rectification transistor MSR. According to aturn-on control signal output by the turn-on control circuit 104 and aturn-off control signal output by the turn-off control circuit 106, theSR flip-flop 202 may output the synchronous rectification control signalVG to control the turn-on state of the synchronous rectificationtransistor MSR.

For example, FIG. 3 is a schematic circuit diagram illustrating asynchronous rectification controller according to another embodiment ofthe disclosure, and FIG. 4 is a schematic signal timing diagram of thesynchronous rectification controller. With reference to FIG. 3 and FIG.4 , the turn-on control circuit 104 may perform a differentiationoperation on the drain voltage signal VD. Further, the turn-on controlcircuit 104 may include a differentiation circuit 302, a comparisoncircuit 304, and an AND gate 306. The differentiation circuit 302 iscoupled between the drain terminal of the synchronous rectificationtransistor MSR and an input terminal of the AND gate 306. The comparisoncircuit 304 is coupled between the drain terminal of the synchronousrectification transistor MSR and another input terminal of the AND gate306. An output terminal of the AND gate 306 is coupled to the setterminal S of the SR flip-flop 202.

The differentiation circuit 302 may perform a differentiation operationon the drain voltage signal VD to obtain a differential signal VD1. Asshown in FIG. 4 , when the power switch Mp is turned on, the inputvoltage VIN supplies power to the coil at the primary side Np of thetransformer T for energy storage. At this time, since the drain voltageVD is rapidly pulled up to K×VIN, a positive surge occurs in thedifferential signal VD1, where K is the coil ratio between the secondaryside Ns and the primary side Np of the transformer T. At this time, theparasitic diode Dr of the synchronous rectification transistor MSR isreversely biased and turned off. In addition, when the power switch Mpis cut off in response to the pulse width modulation signal Spwmgenerated by the pulse width modulation signal generator 110, based onLenz's law, the energy stored at the primary side Np of the transformerT will be transferred to the secondary side Ns of the transformer T. Atthe same time, the parasitic diode Dr of the synchronous rectificationtransistor MSR is forward biased and turned on. Since the body terminalof the synchronous rectification transistor MSR is coupled to the firstground terminal GND1, the voltage level (i.e., the drain voltage VD) ofthe drain terminal of the synchronous rectification transistor MSR dropsrapidly from K×VIN to a negative voltage value. Therefore, a negativesurge occurs in the differential signal VD1.

The differentiation circuit 302 may determine whether the drain voltagesignal VD is greater than or equal to a predetermined voltage Vth (i.e.,whether the voltage of the positive surge is greater than or equal tothe predetermined voltage Vth), and whether a signal value of thedifferential signal VD1 is equal to 0 for a predetermined time Tthduring the period in which the drain voltage signal VD is greater thanor equal to the predetermined voltage Vth (i.e., whether the voltagevalue of the positive surge is maintained at 0 for the predeterminedtime Tth after returning to 0 during the period from the occurrence ofthe positive surge to the occurrence of the negative surge). When thedrain voltage signal VD is greater than or equal to the predeterminedvoltage Vth, and the signal value of the differential signal VD1 isequal to 0 for the predetermined time Tth during the period in which thedrain voltage signal VD is greater than or equal to the predeterminedvoltage Vth, the differentiation circuit 302 outputs a first controlsignal with a high voltage level to the AND gate 306. In addition, ifthe drain voltage signal VD is not greater than or equal to thepredetermined voltage Vth, or the signal value of the differentialsignal VD1 is not equal to 0 for the predetermined time Tth during theperiod in which the drain voltage signal VD is greater than or equal tothe predetermined voltage Vth, the differentiation circuit 302 outputs afirst control signal with a low voltage level to the AND gate 306.

Besides, the comparison circuit 304 may compare the drain voltage signalVD with the turn-on threshold voltage Vonth. When the drain voltagesignal VD is less than or equal to the turn-on threshold voltage Vonth,the comparison circuit 304 outputs a second control signal with a highvoltage level to the AND gate 306. On the other hand, if the drainvoltage signal VD is not less than or equal to the turn-on thresholdvoltage Vonth, the comparison circuit 304 outputs a second controlsignal with a low voltage level to the AND gate 306. The AND gate 306may generate a turn-on control signal to the set terminal S of the SRflip-flop 202 according to the first control signal and the secondcontrol signal.

In addition, the turn-off control circuit 106 may compare the drainvoltage signal VD with the turn-off threshold voltage Voffth. When thedrain voltage signal VD is greater than the turn-off threshold voltageVoffth, the turn-off control circuit 106 may output a turn-off controlsignal with a high voltage level to the reset terminal R of the SRflip-flop 202. On the other hand, if the drain voltage signal VD is notgreater than the turn-off threshold voltage Voffth, the turn-off controlcircuit 106 outputs a turn-off control signal with a low voltage levelto the reset terminal R of the SR flip-flop 202.

Accordingly, if the differentiation circuit 302 determines that thedrain voltage signal VD is greater than or equal to the predeterminedvoltage Vth, and the signal value of the differential signal VD1 isequal to 0 for the predetermined time Tth during the period in which thedrain voltage signal VD is greater than or equal to the predeterminedvoltage Vth, the turn-on control circuit 104 may turn on the synchronousrectification transistor MSR (transform the synchronous rectificationcontrol signal VG to a high voltage level) at the next time when thedrain voltage signal VD is less than or equal to the turn-on thresholdvoltage Vonth. On the other hand, if the differentiation circuit 302determines that the drain voltage signal VD is not greater than or equalto the predetermined voltage Vth, or the signal value of thedifferential signal VD1 is not equal to 0 for the predetermined time Tthduring the period in which the drain voltage signal VD is greater thanor equal to the predetermined voltage Vth, the turn-on control circuit104 does not turn on the synchronous rectification transistor MSR at thenext time when the drain voltage signal VD is less than or equal to theturn-on threshold voltage Vonth, to avoid turning on the synchronousrectification transistor MSR during the oscillation period of the drainvoltage signal VD of the synchronous rectification transistor MSR, whichcauses current reflux at an output terminal of the power conversionapparatus 10, thus causing damage to the power conversion apparatus 10.

It is worth noting that, in some embodiments, the differentiationcircuit 302 may also determine whether to turn on the synchronousrectification transistor MSR according to whether the signal value ofthe differential signal VD1 is greater than or equal to a predeterminedderivative. For example, in the embodiment as shown in FIG. 5 , if thedifferentiation circuit 302 determines that the signal value of thedifferential signal VD1 is greater than or equal to a predeterminedderivative Vrth, and the signal value of the differential signal VD1 ismaintained at 0 for the predetermined time Tth after being reduced fromgreater than or equal to the predetermined derivative Vrth to 0, theturn-on control circuit 104 may turn on the synchronous rectificationtransistor MSR at the next time when the drain voltage signal VD is lessthan or equal to the turn-on threshold voltage Vonth. On the other hand,if the differentiation circuit 302 determines that the signal value ofthe differential signal VD1 is not greater than or equal to thepredetermined derivative Vrth, or the signal value of the differentialsignal VD1 is not maintained at 0 for the predetermined time Tth afterbeing reduced from greater than or equal to the predetermined derivativeVrth to 0, the turn-on control circuit 104 does not turn on thesynchronous rectification transistor MSR at the next time when the drainvoltage signal VD is less than or equal to the turn-on threshold voltageVonth.

FIG. 6 is a schematic circuit diagram illustrating a synchronousrectification controller according to yet another embodiment of thedisclosure, and FIG. 7 is a schematic signal timing diagram of thesynchronous rectification controller of the embodiment of FIG. 6 . Withreference to FIG. 6 and FIG. 7 , the difference between the embodimentof FIG. 6 and the embodiment of FIG. 3 lies in that the differentiationcircuit of FIG. 3 is replaced by an integration circuit 602 in thisembodiment. The integration circuit 602 may perform an integrationoperation on the drain voltage signal VD to obtain an integral signalVD2, and determine whether the drain voltage signal VD is greater thanor equal to the predetermined voltage Vth, and whether a signal value ofthe integral signal VD2 is greater than or equal to a predeterminedintegral Ath during the period in which the drain voltage signal VD isgreater than or equal to the predetermined voltage Vth. Thepredetermined integral Ath may be, for example, an integral value of thedrain voltage signal in a predetermined time T1 during the period inwhich the drain voltage signal VD is greater than or equal to thepredetermined voltage Vth.

When the drain voltage signal VD is greater than or equal to thepredetermined voltage Vth, and the signal value of the integral signalVD2 is greater than or equal to the predetermined integral Ath duringthe period in which the drain voltage signal VD is greater than or equalto the predetermined voltage Vth, the integration circuit 602 outputs afirst control signal with a high voltage level to the AND gate 306. Onthe other hand, if the drain voltage signal VD is not greater than orequal to the predetermined voltage Vth, or the signal value of theintegral signal VD2 is not greater than or equal to the predeterminedintegral Ath during the period in which the drain voltage signal VD isgreater than or equal to the predetermined voltage Vth, the integrationcircuit 602 outputs a first control signal with a low voltage level tothe AND gate 306. The details of implementing the comparison circuit 304and the turn-off control circuit 106 are the same as those in theembodiment of FIG. 3 , so they will not be repeatedly described herein.

Accordingly, if the drain voltage signal VD is greater than or equal tothe predetermined voltage Vth, and the signal value of the integralsignal VD2 is greater than or equal to the predetermined integral Athduring the period in which the drain voltage signal VD is greater thanor equal to the predetermined voltage Vth, the turn-on control circuit104 may turn on the synchronous rectification transistor MSR at the nexttime when the drain voltage signal VD is less than or equal to theturn-on threshold voltage Vonth. On the other hand, if the drain voltagesignal VD is not greater than or equal to the predetermined voltage Vth,or the signal value of the integral signal VD2 is not greater than orequal to the predetermined integral Ath during the period in which thedrain voltage signal VD is greater than or equal to the predeterminedvoltage Vth, the turn-on control circuit 104 does not turn on thesynchronous rectification transistor MSR at the next time when the drainvoltage signal VD is less than or equal to the turn-on threshold voltageVonth, to avoid turning on the synchronous rectification transistor MSRduring the oscillation period of the drain voltage signal VD of thesynchronous rectification transistor MSR.

FIG. 8 is a schematic circuit diagram illustrating a synchronousrectification controller according to still another embodiment of thedisclosure, and FIG. 9 is a schematic signal timing diagram of thesynchronous rectification controller of the embodiment of FIG. 8 . Withreference to FIG. 8 and FIG. 9 , the difference between the embodimentof FIG. 8 and the embodiment of FIG. 3 lies in that the differentiationcircuit of FIG. 3 is replaced by a differentiation-integration circuit802 in this embodiment. The differentiation-integration circuit 802 mayperform a differentiation operation and an integration operation on thedrain voltage signal VD to obtain the differential signal VD1 and theintegral signal VD2, and determine whether the signal value of thedifferential signal VD1 is greater than or equal to the predeterminedderivative Vrth and whether the signal value of the integral signal VD2is greater than or equal to the predetermined integral Ath during theperiod in which the drain voltage signal VD is greater than or equal tothe predetermined voltage Vth.

When the signal value of the differential signal VD1 is greater than orequal to the predetermined derivative Vrth and the signal value of theintegral signal VD2 is greater than or equal to the predeterminedintegral Ath during the period in which the drain voltage signal VD isgreater than or equal to the predetermined voltage Vth, thedifferentiation-integration circuit 802 outputs a first control signalwith a high voltage level to the AND gate 306. On the other hand, if thesignal value of the differential signal VD1 is not greater than or equalto the predetermined derivative Vrth or the signal value of the integralsignal VD2 is not greater than or equal to the predetermined integralAth during the period in which the drain voltage signal VD is greaterthan or equal to the predetermined voltage Vth, thedifferentiation-integration circuit 802 outputs a first control signalwith a low voltage level to the AND gate 306. The details ofimplementing the comparison circuit 304 and the turn-off control circuit106 are the same as those in the embodiment of FIG. 3 , so they will notbe repeatedly described herein.

Accordingly, if the signal value of the differential signal VD1 isgreater than or equal to the predetermined derivative Vrth and thesignal value of the integral signal VD2 is greater than or equal to thepredetermined integral Ath during the period in which the drain voltagesignal VD is greater than or equal to the predetermined voltage Vth, theturn-on control circuit 104 may turn on the synchronous rectificationtransistor MSR at the next time when the drain voltage signal VD is lessthan or equal to the turn-on threshold voltage Vonth. On the other hand,if the signal value of the differential signal VD1 is not greater thanor equal to the predetermined derivative Vrth or the signal value of theintegral signal VD2 is not greater than or equal to the predeterminedintegral Ath during the period in which the drain voltage signal VD isgreater than or equal to the predetermined voltage Vth, the turn-oncontrol circuit 104 does not turn on the synchronous rectificationtransistor MSR at the next time when the drain voltage signal VD is lessthan or equal to the turn-on threshold voltage Vonth, to avoid turningon the synchronous rectification transistor MSR during the oscillationperiod of the drain voltage signal VD of the synchronous rectificationtransistor MSR.

In summary of the foregoing, the turn-on control circuit in theembodiments of the disclosure may perform at least one of adifferentiation operation and an integration operation on the drainvoltage signal of the synchronous rectification transistor, anddetermine whether to turn on the synchronous rectification transistor atthe next time when the drain voltage signal is less than or equal to theturn-on threshold voltage according to at least one of the differentialsignal obtained by performing the differentiation operation and theintegral signal obtained by performing the integration operation.Accordingly, by determining the change of the drain voltage signal ofthe synchronous rectification transistor with the differential signaland the integral signal, the inductor-current discharge period and theinductor-capacitor oscillation period can be accurately distinguished,effectively increasing the accuracy in controlling the switching of thesynchronous rectification transistor to the turn-on state, to thus avoidturning on the synchronous rectification transistor during theinductor-capacitor oscillation period, which causes current reflux at anoutput terminal of the power conversion apparatus, thus causing damageto the power conversion apparatus.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed embodimentswithout departing from the scope or spirit of the disclosure. In view ofthe foregoing, it is intended that the disclosure covers modificationsand variations provided that they fall within the scope of the followingclaims and their equivalents.

What is claimed is:
 1. A synchronous rectification controller, configured to control a turn-on state of a synchronous rectification transistor, the synchronous rectification controller comprising: a turn-on control circuit, coupled to a drain terminal of the synchronous rectification transistor, performing at least one of a differentiation operation and an integration operation on a drain voltage signal of the synchronous rectification transistor, and determining whether to turn on the synchronous rectification transistor at a next time when the drain voltage signal is less than or equal to a turn-on threshold voltage according to at least one of a differential signal obtained by performing the differentiation operation and an integral signal obtained by performing the integration operation; and a turn-off control circuit, coupled to the drain terminal of the synchronous rectification transistor, comparing the drain voltage signal with a turn-off threshold voltage, and turning off the synchronous rectification transistor when the drain voltage signal is greater than the turn-off threshold voltage, wherein if the drain voltage signal is greater than or equal to a predetermined voltage, and a signal value of the differential signal is equal to 0 for a predetermined time during a period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit turns on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage; and if the drain voltage signal is not greater than or equal to the predetermined voltage, or the signal value of the differential signal is not equal to 0 for the predetermined time during the period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit does not turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage.
 2. The synchronous rectification controller of claim 1, further comprising: a logic circuit, coupled to the turn-on control circuit, the turn-off control circuit, and a gate terminal of the synchronous rectification transistor, and controlled by the turn-on control circuit and the turn-off control circuit to generate a synchronous rectification control signal to the gate terminal of the synchronous rectification transistor.
 3. The synchronous rectification controller of claim 2, wherein the logic circuit comprises an SR flip-flop, a set terminal and a reset terminal of the SR flip-flop are respectively coupled to the turn-on control circuit and the turn-off control circuit, and an output terminal of the SR flip-flop is coupled to the gate of the synchronous rectification transistor terminal.
 4. The synchronous rectification controller of claim 1, wherein the turn-off threshold voltage is greater than the turn-on threshold voltage.
 5. A synchronous rectification controller, configured to control a turn-on state of a synchronous rectification transistor, the synchronous rectification controller comprising: a turn-on control circuit, coupled to a drain terminal of the synchronous rectification transistor, performing at least one of a differentiation operation and an integration operation on a drain voltage signal of the synchronous rectification transistor, and determining whether to turn on the synchronous rectification transistor at a next time when the drain voltage signal is less than or equal to a turn-on threshold voltage according to at least one of a differential signal obtained by performing the differentiation operation and an integral signal obtained by performing the integration operation; and a turn-off control circuit, coupled to the drain terminal of the synchronous rectification transistor, comparing the drain voltage signal with a turn-off threshold voltage, and turning off the synchronous rectification transistor when the drain voltage signal is greater than the turn-off threshold voltage, wherein if a signal value of the differential signal is greater than or equal to a predetermined derivative, and the signal value of the differential signal is maintained at 0 for a predetermined time after being reduced from greater than or equal to the predetermined derivative to 0, the turn-on control circuit turns on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage; and if the signal value of the differential signal is not greater than or equal to the predetermined derivative, or the signal value of the differential signal is not maintained at 0 for the predetermined time after being reduced from greater than or equal to the predetermined derivative to 0, the turn-on control circuit does not turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage.
 6. A synchronous rectification controller, configured to control a turn-on state of a synchronous rectification transistor, the synchronous rectification controller comprising: a turn-on control circuit, coupled to a drain terminal of the synchronous rectification transistor, performing at least one of a differentiation operation and an integration operation on a drain voltage signal of the synchronous rectification transistor, and determining whether to turn on the synchronous rectification transistor at a next time when the drain voltage signal is less than or equal to a turn-on threshold voltage according to at least one of a differential signal obtained by performing the differentiation operation and an integral signal obtained by performing the integration operation; and a turn-off control circuit, coupled to the drain terminal of the synchronous rectification transistor, comparing the drain voltage signal with a turn-off threshold voltage, and turning off the synchronous rectification transistor when the drain voltage signal is greater than the turn-off threshold voltage, wherein if the drain voltage signal is greater than or equal to a predetermined voltage, and a signal value of the integral signal is greater than or equal to a predetermined integral during a period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit turns on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage; and if the drain voltage signal is not greater than or equal to the predetermined voltage, or the signal value of the integral signal is not greater than or equal to the predetermined integral during the period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit does not turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage.
 7. The synchronous rectification controller of claim 6, wherein the predetermined integral is equal to an integral value of the drain voltage signal in a predetermined time during the period in which the drain voltage signal is greater than or equal to the predetermined voltage.
 8. A synchronous rectification controller, configured to control a turn-on state of a synchronous rectification transistor, the synchronous rectification controller comprising: a turn-on control circuit, coupled to a drain terminal of the synchronous rectification transistor, performing at least one of a differentiation operation and an integration operation on a drain voltage signal of the synchronous rectification transistor, and determining whether to turn on the synchronous rectification transistor at a next time when the drain voltage signal is less than or equal to a turn-on threshold voltage according to at least one of a differential signal obtained by performing the differentiation operation and an integral signal obtained by performing the integration operation; and a turn-off control circuit, coupled to the drain terminal of the synchronous rectification transistor, comparing the drain voltage signal with a turn-off threshold voltage, and turning off the synchronous rectification transistor when the drain voltage signal is greater than the turn-off threshold voltage, wherein if a signal value of the differential signal is greater than or equal to a predetermined derivative and a signal value of the integral signal is greater than or equal to a predetermined integral during a period in which the drain voltage signal is greater than or equal to a predetermined voltage, the turn-on control circuit turns on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage; and if the signal value of the differential signal is not greater than or equal to the predetermined derivative or the signal value of the integral signal is not greater than or equal to the predetermined integral during the period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit does not turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage.
 9. The synchronous rectification controller of claim 8, wherein the predetermined integral is equal to an integral value of the drain voltage signal in a predetermined time during the period in which the drain voltage signal is greater than or equal to the predetermined voltage.
 10. A power conversion apparatus, comprising: a transformer, having a primary side and a secondary side, wherein a first terminal of the primary side is configured to receive an input voltage, and a first terminal of the secondary side is configured to provide an output voltage to a load; a synchronous rectification transistor, wherein a drain terminal of the synchronous rectification transistor is coupled to a second terminal of the secondary side, and a source terminal of the synchronous rectification transistor is coupled to a ground terminal; and a synchronous rectification controller, coupled between the drain terminal and a gate terminal of the synchronous rectification transistor, configured to control a turn-on state of the synchronous rectification transistor, and comprising: a turn-on control circuit, coupled to the drain terminal of the synchronous rectification transistor, performing at least one of a differentiation operation and an integration operation on a drain voltage signal of the synchronous rectification transistor, and determining whether to turn on the synchronous rectification transistor at a next time when the drain voltage signal is less than or equal to a turn-on threshold voltage according to at least one of a differential signal obtained by performing the differentiation operation and an integral signal obtained by performing the integration operation; and a turn-off control circuit, coupled to the drain terminal of the synchronous rectification transistor, comparing the drain voltage signal with a turn-off threshold voltage, and turning off the synchronous rectification transistor when the drain voltage signal is greater than the turn-off threshold voltage, wherein if the drain voltage signal is greater than or equal to a predetermined voltage, and a signal value of the differential signal is equal to 0 for a predetermined time during a period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit turns on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage; and if the drain voltage signal is not greater than or equal to the predetermined voltage, or the signal value of the differential signal is not equal to 0 for the predetermined time during the period in which the drain voltage signal is greater than or equal to the predetermined voltage, the turn-on control circuit does not turn on the synchronous rectification transistor at the next time when the drain voltage signal is less than or equal to the turn-on threshold voltage. 